This invention relates to the treatment of wells penetrating subterranean formations and more particularly to the isolation of an interval within a well for the introduction of a treating fluid into an adjacent formation.
Various treatment procedures are known in the art for the treatment of a well penetrating a subterranean formation. One common treatment procedure involves the hydraulic fracturing of a subterranean formation in order to increase the flow capacity thereof. Thus, in the oil industry, it is a conventional practice to hydraulically fracture a well in order to produce fractures or fissures in the surrounding formations and thus facilitate the flow of oil and/or gas into the well from the formation or the injection of fluids from the well into the formation. Such hydraulic fracturing can be accomplished by disposing a suitable fracturing fluid within the well opposite the formation to be fractured. The well is open to the formation by virtue of openings in a conduit, such as a casing string, or by virtue of an open completion in which a casing string is set to the top of the desired open interval and the formation face then exposed directly to the well below the shoe of the casing string. In any case, sufficient pressure is applied to the fracturing fluid and to the formation to cause the fluid to enter into the formation under a pressure sufficient to break down the formation with the formation of one or more fractures. Oftentimes the formation is ruptured to form vertical fractures. Particularly, in relatively deep formations, the fractures are naturally oriented in a predominantly vertical direction. One or more fractures may be produced in the course of a fracturing operation, or the same well may be fractured several times at different intervals in the same or different formation.
Another widely used treating technique involves acidizing, which is generally applied to calcareous formations such as limestone. In acidizing, an acidizing fluid such as hydrochloric acid is introduced into the well and into the interval of the formation to be treated which is exposed in the well. Acidizing may be carried out as so-called xe2x80x9cmatrix acidizingxe2x80x9d procedures or as xe2x80x9cacid fracturingxe2x80x9d procedures. In acid fracturing, the acidizing fluid is injected into the well under a sufficient pressure to fracture the formation in the manner described previously. An increase in permeability in the formation adjacent the well is produced by the fractures formed in the formation as well as by the chemical reaction of the acid with the formation material. In matrix acidizing, the acidizing fluid is introduced through the well into the formation at a pressure below the breakdown pressure of the formation. In this case, the primary action is an increase in permeability primarily by the chemical reaction of the acid within the formation with there being little or no effect of a mechanical disruption of the formation, such as occurs in hydraulic fracturing.
Various other treatment techniques are available for increasing the permeability of a formation adjacent a well or otherwise imparting a desired characteristic to the formation. For example, solvents can sometimes be involved as a treating fluid in order to remove unwanted material from the formation in the vicinity of the well bore.
In accordance with the present invention, there is provided a method for the treatment of a subterranean formation penetrated by a well. In carrying out the invention, first and second flow paths are established within the well, extending from the wellhead into the vicinity of the subterranean formation. A plugging fluid comprising a suspension of a particulate plugging agent in a carrier liquid is circulated into the first of the flow paths and into the well in contact with the wall of the well within the subterranean formation. The carrier liquid is separated from the particulate plugging agent by circulating the carrier liquid into a second flow path. Circulation of the liquid is accomplished through a set of openings leading to the second flow path, which are dimensioned to allow the passage of the carrier liquid while retaining the particulate plugging agent in contact with the set of openings. The circulation of the plugging fluid continues until the particulate plugging agent accumulates to form a bridge packing within the well. The bridge packing acts similarly as a mechanical packer to form a barrier within the well. Subsequent to establishing the bridge packing, a treating fluid is introduced into the well through the first flow path and in contact with the surface of the formation in the well adjacent to the accumulated plugging agent forming the bridge packing.
In a further aspect of the invention, a treatment procedure is carried out in a section of a well penetrating a subterranean formation and having a return tubing string provided with spaced screened sections at a location in the well adjacent the subterranean formation. A working tubing string opens into the interior of the well intermediate the spaced screen sections. In carrying out the invention, a plugging agent comprising a suspension of particulate plugging agent in a carrier liquid is circulated through the working string into the intermediate interval between the screen sections. The carrier liquid is flowed through openings in the spaced screen section, which are sized to allow the passage of the carrier liquid while retaining the particulate plugging agent in the well in contact with the screen sections. The flow of the plugging agent within the well is continued until the particulate plugging agent in the fluid accumulates in the well adjacent the screen sections to form spaced bridge packings within the well and surrounding the return string. Thereafter, a treating fluid is introduced into the well and into the interval of the well intermediate the spaced bridge packings and introduced into the formation. In a specific application of the invention, the treating fluid is a fracturing fluid introduced into the treating interval under pressure sufficient to hydraulically fracture the formation. In another embodiment of the invention, the treating fluid is an acidizing fluid effective to acidize the formation in either a matrix acidizing or acid fracturing operation. Preferably, subsequent to the introduction of the treating fluid into the well, a clean-up fluid is circulated down the well into the return tubing string to displace the accumulated particulate plugging agent away from the screened sections and disrupt and remove the bridge packings. In carrying out the hydraulic fracturing operations, the fracturing fluid is normally in the nature of a cross-linked gel having a high viscosity. The clean-up fluid can incorporate a breaker to break down the viscosifying agent in the fracturing fluid. For example, where the viscosifier in an aqueous-based fracturing agent takes the form of hydroxethylcellulose, the clean-up fluid can incorporate an acid such as hydrochloric acid, which functions to break the fracturing fluid gel to a liquid of much lower viscosity. Subsequently, the tubing strings can be moved longitudinally through the well to a second location within the well bore spaced from the originally treated location and the operation then repeated to treat a different section of the well bore. The tubing strings employed in carrying out the invention may be parallel tubing strings or they may be concentrically oriented tubing strings in which the working string disposed within the return string provides a return pathway formed by the annulus of the working string and the return string.
In a further application of the invention, a treating process is carried out in a well section that extends in a horizontal orientation within the subterranean formation. The fracturing operation is carried out to hydraulically fracture the formation and form a vertically oriented fracture within the formation extending from the horizontally oriented well bore. Thereafter, the return and working strings are moved longitudinally through the horizontally extending well section to a second location, and the operation is repeated to form a second set of bridge packings followed by hydraulic fracturing to form a second vertically oriented fracture within the well section spaced at some distance from the initially formed vertically oriented fracture. These operations can be repeated as many times as desired in order to produce multiple fractures.